The general form is fully clarified for BRS-invariant local operators O in gauge theories. Any O with negative FP-ghost number NFP≪0 is proved to be null-operator, i.e., operator expressible as the BRS-transform of another operator. Any O with NFP=0 is given by gauge-invariant operator plus null-operator. Any O with NFP>0 has a form similar to the NFP=0 case; the structure of its non-null-operator part is also completely clarified.